Motor gasolines are undergoing ever changing formulations in order to reduce the level of potentially environmentally damaging components and to meet new government legislation. One requirement is the reduction of benzene in gasoline down to very low levels, for example less than one percent. Other applications may involve removing benzene from a variety of blending, product, or effluent waste streams.
In conventional refining processes producing gasoline streams, the resulting stream typically contains 2-3 percent benzene. While various techniques can be used to selectively remove this benzene, the use of solid adsorbents, such as molecular sieves, presents advantages over other techniques such as distillation and solvent extraction. Distillation is not suitable primarily because benzene forms low boiling azeotropes with normal hexane and naphthenes such as methyl cyclopentane and cyclohexane. Alternatively, extraction of the benzene with a solvent, such as sulfolane, is technically feasible but presents some disadvantages including the use of special equipment to compensate for the corrosive nature of the sulfolane.
It is often the desorption step that presents difficulties in adsorption based separation processes. The extent to which a desorbent will displace an adsorbed material depends upon the relative strength of adsorption of the desorbent over the adsorbed material. Desorption of aromatics from solid adsorbents such as zeolites is particularly difficult as aromatic molecules tend to be very strongly adsorbed relative to paraffinic, naphthenic and olefinic molecules. Thus adsorbed aromatics, such as benzene, are usually desorbed using a large excess of desorbent such as toluene or xylene. Even so, toluene and xylene often do not readily displace benzene leading to long desorption times.
U.S. Pat. Nos. 3,207,803 and 3,243,470 disclose a process for the selective removal of straight-chain, olefinic hydrocarbons from an admixture with branched and cyclic hydrocarbons by adsorbing the unsaturated hydrocarbons onto a modified zeolite. Hydrogen is introduced, either simultaneously with the feed or following the feed, to hydrogenate the adsorbed unsaturated hydrocarbons to more saturated hydrocarbons in order, it is stated, to facilitate their removal from the zeolite. These patents neither mention nor suggest the advantages of first hydrogenating adsorbed aromatic compounds to facilitate their desorption.
Recently filed U.S. patent application Ser. No. 729,679, filed Jul. 15, 1992, relates to selective removal of benzene from process streams but does not mention conversion of the benzene to cyclohexane.